EE 122: Lecture 24 (Security) Ion Stoica December 4, 2001

Security Requirements  Authentication -Ensures that the sender and the receiver are who they are claiming to be  Data integrity -Ensure that data is not changed from source to destination  Confidentiality -Ensures that data is red only by authorized users  Non-repudiation -Ensures that the sender has strong evidence that the receiver has received the message, and the receiver has strong evidence of the sender identity, strong enough such that the sender cannot deny that it has sent the message and the receiver cannot deny that it has received the message (not discussed in this lecture)

Cryptographic Algorithms  Security foundation: cryptographic algorithms -Secret key cryptography, Data Encryption Standard (DES) -Public key cryptography, RSA algorithm -Message digest, MD5

Symmetric Key  Both the sender and the receiver use the same secret keys Internet Encrypt with secret key Decrypt with secret key Plaintext Ciphertext

Data Encryption Standard (DES)  DES encrypts a 64-bit block of plain text using a 64-bit key  Three phases 1.Permute the 64 bits in the block 2.Apply a given operation 16 times on the 64 bits 3.Permute the 64 bits using the inverse of the original permutation Round 1 Round key 1 st phase 3 rd phase 2 nd phase

Operation in Each Round of 2 nd Phase  Key is 56 bits  Each round the key is modified and 48 bits are selected from it. Given result K i, the following operations are performed + F KiKi L i-1 R i-1 LiLi RiRi

Encrypting Larger Messages  Initialization Vector (IV) is a random number generated by sender and sent together with the ciphertext + Block 1 Cipher 1 DES + Block 2 DES + Block 3 DES + Block 4 DES Cipher 2 Cipher 3 Cipher 4 IV

Discussion  Can provide confidentiality  No mathematical proof, but practical evidence suggests that decrypting a message without knowing the key requires exhaustive search  To increase security use triple-DES, i.e., encrypt the message three times

RSA (Rivest, Shamir, and Adleman)  Sender uses a public key  Receiver uses a private key Internet Encrypt with public key Decrypt with private key Plaintext Ciphertext

Generating Public and Private Keys  Choose two large prime numbers p and q (~ 256 bit long) and multiply them: n = p*q  Chose encryption key e such that e and (p-1)*(q-1) are relatively prime  Compute decryption key d, where d = e -1 mod ((p-1)*(q-1))  Construct public key from pair (n, e)  Construct private key from pair (d, n)

RSA Encryption and Decryption  Encryption: -c = m e mod n  Decryption: -m = c d mod n

Example  Choose p = 7 and q = 11  n = p*q = 77  Compute encryption key e: (p-1)*(q-1) = 6*10 = 60  chose e = 13 (13 and 60 are relatively prime numbers)  Compute decryption key d: d = mod 60  13*d mod 60 = 1 mod 60  d = 37 (37*13 = 481)

Example (cont’d)  n = 77; e = 13; d = 37  Send message m = 7  Encryption: c = m e mod n = 7 13 mod 77 = 35  Decryption: m = c d mod n = mod 77 = 7

Discussion  Can provide confidentiality  A receiver A computes n, e, d, and sends out (n, e) -Everyone who wants to send a message to A uses (n, e) to encrypt it  How difficult is to recover d ? (Someone that can do this can decrypt any message sent to A !)  Recall that d = e -1 mod ((p-1)*(q-1))  So to find d, you need to find primes factors p and q -This is provable very difficult

Message Digest (MD) 5  Can provide data integrity -Used to verify the authentication of a message  Idea: compute a hash on the message and send it along with the message  Receiver can apply the same hash function on the message and see whether the result coincides with the received hash

MD 5 (cont’d)  Basic property: digest operation very hard to invert - in practice someone cannot alter the message without modifying the digest Internet Digest (MD5) Plaintext digest Digest (MD5) = digest’ NO corrupted msg Plaintext

Message Digest Operation  Transformation contains complex operations (see textbook, page 583) 512 bits Message (padded) Initial digest (constant) Transformation... Message digest

Authentication  Goal: Make sure that the sender an receiver are the ones they claim to be  Two solutions based on secret key cryptography (e.g., DES) -Three-way handshaking -Trusted third party  One solution based on public key cryptography (e.g., RSA) -Public key authentication

Simple Three-Way Handshaking  E(m,k) – encrypt message m with key k  D(m,k) – decrypt m with key k  CHK and SHK – client and server shared secrete keys  SK – session key used for data communication -This reduces the number of messages containing CHK and SHK  Question: how are CHK and SHK communicated in the first place? clientId, E(x, CHK) E(x+1, SHK), E(y,SHK) E(y+1, CHK) E(SK,SHK) client server

Trusted Third Party  Trust a third party entity, authentication server -E.g., Kerberos protocol  Scenario: A wants to communicate with B  Assumption: both A and B share secrete keys with S: K A and K B  Notations: -T: timestamp (also serves the purpose of a random number) -L: lifetime of the session -K: session’s key

Trusted Third Party (cont’d) A,B E(T+1,K) E((T,L,K,B),K A ) E((T,L,K,A),K B ) E((A,T),K A ) E((T,L,K,A),K B ) S AB

Public Key Authentication  Based on public key cryptography  Each side need only to know the other side’s public key -No secrete key need to be shared  A encrypts a random number x and B proves that it knows x  A can authenticate itself to be in the same way E(x, Public B ) x A B